Neuronal development in the Drosophila eye follows a non-clonal mechanism where cells are instructed by their microenvironment to take on the fate of identified neurons. The sevenless gene participates in this process and mediates the development of the R7 photoreceptor neuron. We have isolated a dominant second site suppressor of the sevenless phenotype. It is called Suppressor of sevenless (Sos). We have mapped this mutation and shown that the suppression is specific to the E4 allele of sevenless, that makes a full sized protein product. We propose that the mutant product of the Sos gene is compensating for the defect in the sevE4 protein, and that in wild type flies, the product of the sevenless gene has a function relevant to the sevenless pathway. Techniques from Drosophila genetics and molecular biology will be used for detailed characterization of the Sos gene. The aspect of sevenless function that the suppressor interacts with will be determined by molecular analysis of the sevenless allele that is suppressed. Genetic mosaic studies will identify the cell(s) responsible for the suppressor function. The Sos gene will be cloned and characterized. From a detailed study of the suppressor, we hope to obtain a better understanding of a pathway that leads to neuronal differentiation.